The new Band Aid single is released in record stores on Monday, with staff poised for a massive rush in sales.
The 2004 version of Do They Know It's Christmas? is firm favourite to be this year's Christmas number one.
HMV is opening the doors of its flagship London store one hour earlier, at 0800 GMT, to allow people to buy copies on their way to work.
HMV's Gennaro Castaldo said it could sell a million "if it captures the imagination like the original".
He added: "In 1984, people bought multiple copies to give to family and friends as presents. If the same happens this year, sales could even exceed two million."
The original version of the charity track sold 750,000 copies in its first week and 3.5 million in total.
The last million-selling singles in the UK were by Pop Idol stars Will Young and Gareth Gates in 2002.
"Band Aid 20 is easily the biggest release since Elton John's Candle In The Wind tribute to Princess Diana," said Mr Castaldo.
"It looks an absolute cert to be the Christmas number one and could stay at the top for six weeks, if not longer."
Robbie Williams is one of the artists featured on Band Aid 20
The latest version features artists including Dido, Robbie Williams, Chris Martin and Dizzee Rascal. It also features Bono reprising his original lyric from the 1984 version.
But users of the world's most popular online music store, Apple's iTunes, cannot download the track.
Other download services have agreed to raise their usual song prices and give proceeds to the Band Aid Trust charity, which is raising money for food aid in Darfur, Sudan.
But Apple was unwilling to do the same, according to Band Aid organisers.
HMV has joined Virgin and Woolworths to donate their profits, while the government is to refund the VAT paid on the single and recently-released Live Aid DVD.document.write(unescape("